Over the last ten years, a general effort towards miniaturization of the analytical tools has been observed. Two main reasons are pushing the development miniaturized chemical apparatus, which have been called Micro Total Analysis Systems (μ-TAS): a decrease of analyte consumption and a decrease of duration of single analysis. Both needs are particularly evident in the new development of life science, where genetic analysis and high throughput screening in drug discovery take more and more importance. In these applications, the reason for limiting the analyte consumption are evidenced by the increasing number of performed assays. In this case, the use of reactants for analysis must be as small as possible in order not only to reduce the cost but also to limit the waste production. In other cases, the analysis of extremely small volumes is required. Such a volume may be only a few nL, e.g. in the case of neurological fluid analysis or in prenatal diagnostics. In many cases, the decrease in analysis time is also an important issue e.g. in medical diagnostics, where the time factor may signify a fatal issue for the patient. Two different and complementary strategies have been developed in parallel to achieve these goals. On one hand, the fabrication of microfluidic devices has allowed fluid handling in pL volumes and, on the other hand, immobilization of affinity reagents into high density 2-dimensional arrays for high throughput affinity analysis.
In recent years, capillary electrophoretic methods have enjoyed gaining popularity, primarily due to the observed high separation efficiencies, peak resolution, and wide dynamic ranges of molecular weights that may be analyzed. Furthermore, the simple open-tubular capillary design has lead itself to a variety of automation, injection and detection strategies developed previously for more conventional analytical technologies.
The general instrumental set-up involves a capillary filled with an electrolyte solution and a high voltage power supply connected to electrodes in contact with small fluid filled reservoirs at either end of the capillary. The power supply is operated in order to apply an electric potential field tangential to the capillary surface, in the range of 100–1000 V/cm. When the potential is applied, migration processes occur. The electric field imposes a force onto charged species leading to the electrophoretic migration of sample molecules within the capillary. Furthermore, when file capillary surface is charged, a flow of the whole solution is induced by electro-osmosis. Therefore, electrophoresis is in most cases superimposed on a so-called electroosmotic flow (EOF). Species moving in the capillary as a result of these forces will then be transported past a suitable detector, absorbance and fluorescence being the most common. Capillary electrophoresis has been applied to numerous analytes spanning pharmaceutical, environmental and agricultural interests. A common focus amongst these activities is bioanalysis. Separation methods are developed for peptide sequencing, amino acids, isoelectric point determination for proteins, enzyme activity, nucleic acid hybridization, drugs and metabolites in biological matrices and affinity techniques such as immunoassays. Furthermore, buffer additives such as cyclodextrins and micellar phases have added the ability to perform chiral separations of biologically active enantiomers of tryptophan derivatives, ergot alkaloids, epinephrines and others which is of great interest to the pharmaceutical industry.
The capillaries described above generally have internal diameters between 50–200 μm and are formed in fused silica. The microfabrication of capillaries has also been accomplished by machining directly onto planar, silicon-based substrates. Silicon substrates have an abundance of charged silanol groups and thus generate considerable EOF. In the case of micromachining, EOF can be an advantage in that the flow of the bulk solution can be used for many liquid handling operations. There has recently been intense activity in the area of chemical instrumentation miniaturization. Efforts have been made to reduce whole laboratory systems on to microchip substrates, and these systems have been termed micro-Total Analytical Systems (μ-TAS). As already mentioned, most of such μ-TAS devices to date have been produced photolithographicly on silicon-based substrates. This process involves the generation of the desired pattern on a mask, through which a photoresist coated silicon dioxide wafer is exposed to light. Solubilised photoresist is then removed and the resulting pattern anisotropically etched with hydrofluoric acid. Etched capillaries are then generally sealed by thermal bonding with a glass covert The bonding technique in particular is labour and technology intensive and thermal bonding requires temperatures between 600–1000° C. This bonding technique has a very low tolerance of defect or presence of dust and requires clean room conditions for the fabrication, which means that the production is very expensive. Alternative fabrication techniques have also been developed based on organic polymers. Fabrication of polymer microfluidic devices has been shown by injection moulding or polymerising polydimethyl siloxane (PDMS) on a mould. These two techniques have the advantage to replicate a large number of micro-structures with the same pattern given by the mould. Other techniques based on electromagnetic radiation either for polymerisation under X-ray (LIGA) or for ablation have also been recently shown to be feasible. This last fabrication technique allows fast prototyping by writing pattern on a substrate that can be moved in the X and Y directions. Different structures can then be fabricated just by moving the substrate in front of the laser beam.
As already mentioned, electroosmotic pumping is used here not only to separate samples but also to dispense discrete amounts of reactants or to put in contact solutions for the reaction in continuous flow systems. A large diversity of structures and electrical connections have been presented which permit to deliver and analyse samples in less than a millisecond by electrophoresis for example.
This spectacular property also evidences that, in these microchannels, the main transport mechanism between two flowing solutions is diffusion. As different species exhibit different diffusion coefficients, efficient mixing becomes problematic, and this is often presented as a serious limitation for the wider use of microfluidic in total analysis systems. In order to solve this problem, mixers have been presented, where the flows are for instance divided in smaller channels (20 μm) before being placed in contact. In this manner, the diffusion time is reduced and hence the mixing efficiency enhanced.
Many recent advances in chemical analysis have involved the incorporation of biomolecules capable of selective and high affinity binding to analytes of interest Such devices are often termed biosensors, which involve real-time transduction of the binding event into an electronic signal, but also include analytical technology consisting of immunoassays, enzyme reaction, as well as nucleic acid hybridisation. Bio-analytical devices utilising this technology have been applied to a wide range of applications in medicine agriculture, industrial hygiene, and environmental protection. Enzyme electrodes represent the oldest group of biosensors and are being increasingly used for clinical testing of metabolites such us glucose, lactate, urea, creatinine or bilirubin. Several groups have developed needle-type electrodes. for subcutaneous glucose measurements. A microelectrochemical enzyme transistor has been developed for measuring low concentrations of glucose. Efforts continued towards other clinically relevant metabolites particularly for the multiple-analyte determination. Strategy to incorporate affinity steps is also an active area of biosensors. The emerging area of DNA hybridisation biosensors has been a very popular topic for the clinical diagnosis of inherited diseases and for the rapid detection of infectious microorganisms.
Recent interest in the development of miniaturised, array-based multianalyte binding assay methods suggests that the ligand assay field is on the brink of a technological revolution. The studies in this area have centered largely on antibody or DNA spot arrays localised on microchips which are potentially capable of determining the amounts of hundreds of different analytes in a small sample (such as a single drop blood). Array-based immunoassay methods shows the particular importance in areas such as environmental monitoring where the concentrations of many different analytes in test samples are required to be simultaneously determined. Affymetrix developed ways to synthesise and assay biological molecules in a highly dense parallel formal Integration of two key technologies forms the cornerstone of the method. The first technology, light-directed combinatorial chemistry, enables the synthesis of hundreds of thousands of discrete compounds at high resolution and precise locations on a substrate. The second laser confocal fluorescence scanning permits measurement of molecular interactions on the array.
Recently, the Laboratoire d'Electrochimie of the EPFL Lausanne has presented a patterning technique based on the photoablation process. In order to fabricate microarrays of proteins, the polymer substrate is firstly blocked with a bovine serum albumin (BSA) layer avoiding non specific adsorption of protein on the substrate layer. Microspots are then created on the surface by photoablation of the BSA layer, on which avidin can be adsorbed yet. This micropatterning technique allows then to specifically adsorb antibodies linked to biotin on the avidin spots as visualised by biotin-fluorescein complex.
Apart from electrophoretic separations and hybridisation, an increasing number of applications on μ-TAS have been shown in the last few years. Full DNA analysers have been implemented in a single device with a polymerase chain reaction (PCR) chamber followed by an electrophoretic separation. Continuous flow PCR has also been shown where the analyte solution is driven through capillary crossing zones of different temperatures. Other genetic analysis have also been demonstrated comprising high speed DNA sequencing, high density parallel separation or single DNA molecule detection. Another application of μ-TAS has been shown in electrochromatography. An open-channel electrochromatography in combination with solvent programming has been demonstrated using a microchip device. Others have successfully used μ-TAS to conduct immunoassays involving competitive markers, noting several advantages over more traditional formats including (a) high efficiency separations between competitive markers and antibody-marker complexes, (b) excellent detection limits (0.3–0.4 amol injected) at high speed, and (c) good potential for automation. This has first been demonstrated in a micromachined capillary electrophoresis device by Koutny et al. Cortisol was determined in serum using a competitive immunoassay that was subsequently quantitated using μ-TAS. A microfluidic system was fabricated on a glass chip to study immobilization of biological cells on-chip. Electroosmotic and/or electrophoretic pumping were used to drive the cell transport within a network of capillary channels. An automated enzyme assay was performed within a microfabricated channel network. Precise concentrations of substrate, enzyme and inhibitor were mixed in nanoliter volumes using electrokinetic flow. Finally, the new insight in the use microfabricated system has been to combine the advantage of parallel reactions and liquid handling in extremely small volumes with an electrospray or nanospray interface for mass spectrometry analysis. This last application opens a way to efficiently use the microchip format not only for genetic analysis where it is already recognised but also in protein sequencing.
Several microfabrication processes have been shown that modify the surface properties of the polymer.
It is known that reactions of gas plasmas with polymers can be classified as follows:
1. Surface reactions:                Reactions between the gas-phase species and surface species produce functional groups and/or crosslinking sites at the surface.        
2. Plasma polymerisation:                The formation of a thin film on the surface of a polymer via polymerisation of an organic monomer such as CH4, C2H6, C2F4 and C3F6 in a plasma.        
3. Cleaning and etching:                Materials are removed from a polymer surface by chemical reactions and physical etching at the surface to form volatile product.        
Patent of particular relevance in the etching process:
U.S. Pat. No. 5,099,299 (Dyconex)
Patent with particular relevance in lamination sealing of polymer micro-structure:
WO 991197 17 (Aclara Biosciences)
Patent of particular relevance in patterning of properties:
WO 9823957 A(EPFL)
Other patents on microfabrication and fluidic control by surface properties:
WO 9823957 A(EPFL)
WO 9846439 (Caliper technology)
WO 9807019 (Gamera Bioscience)